---
title: Constructing Types
---

For many apps, you can define a fixed schema when the application starts, and define it using GraphQL schema language. In some cases, it's useful to construct a schema programmatically. You can do this using the `GraphQLSchema` constructor.

When you are using the `GraphQLSchema` constructor to create a schema, instead of defining `Query` and `Mutation` types solely using schema language, you create them as separate object types.

For example, let's say we are building a simple API that lets you fetch user data for a few hardcoded users based on an id. Using `buildSchema` we could write a server with:

```js
import express from "express";
import { createHandler } from "graphql-http/lib/use/express";
import { buildSchema } from "graphql";

const schema = buildSchema(`
  type User {
    id: String
    name: String
  }

  type Query {
    user(id: String): User
  }
`);

// Maps id to User object
const fakeDatabase = {
  a: {
    id: "a",
    name: "alice",
  },
  b: {
    id: "b",
    name: "bob",
  },
};

const root = {
  user({ id }) {
    return fakeDatabase[id];
  },
};

const app = express();
app.all(
  "/graphql",
  createHandler({
    schema,
    rootValue: root,
  })
);
app.listen(4000);
console.log("Running a GraphQL API server at localhost:4000/graphql");
```

We can implement this same API without using GraphQL schema language:

```js
import express from "express";
import { createHandler } from "graphql-http/lib/use/express";
import {
  GraphQLObjectType,
  GraphQLString,
  GraphQLSchema,
} from "graphql";

// Maps id to User object
const fakeDatabase = {
  a: {
    id: "a",
    name: "alice",
  },
  b: {
    id: "b",
    name: "bob",
  },
};

// Define the User type
const userType = new GraphQLObjectType({
  name: "User",
  fields: {
    id: { type: GraphQLString },
    name: { type: GraphQLString },
  },
});

// Define the Query type
const queryType = new GraphQLObjectType({
  name: "Query",
  fields: {
    user: {
      type: userType,
      // `args` describes the arguments that the `user` query accepts
      args: {
        id: { type: GraphQLString },
      },
      resolve: (_, { id }) => {
        return fakeDatabase[id];
      },
    },
  },
});

const schema = new GraphQLSchema({ query: queryType });

const app = express();
app.all(
  "/graphql",
  createHandler({
    schema,
  })
);
app.listen(4000);
console.log("Running a GraphQL API server at localhost:4000/graphql");
```

When we use this method of creating the API, the root level resolvers are implemented on the `Query` and `Mutation` types rather than on a `root` object.

This is particularly useful if you want to create a GraphQL schema automatically from something else, like a database schema. You might have a common format for something like creating and updating database records. This is also useful for implementing features like union types which don't map cleanly to ES6 classes and schema language.
